Yes, you can use graphite on enamel. The basic process is to use a light colored enamel on metal and draw on it with a graphite pencil.
It’s not quite that simple. If you have ever tried to draw on glass you know that the pencil point will slide all over without leaving a mark. The idea is to create a matte finish or ‘tooth’ on the enamel that will snag the graphite. Usually, this can be done by stoning the surface or by using an etching cream. Neither technique provides instant gratification. Stoning is difficult to get a smooth even surface and etching takes time and exposure to chemicals.
I like to experiment with different things combined with enamels, just to see what it will do. I’ve used salt, sugar, sand, baking soda, vinegar, dawn dish soap and milk. While experimenting I found out that if you paint milk on to your enamel and let it dry (I use a heat gun, I hate to wait) it enables you to draw on it with a pencil, no chemicals, no elbow grease.
I find that heavy cream works the best. Paint it on, let it dry, draw on it, and fire. I use a Smith mini-torch with propane and oxygen to fire all my enamels.